This invention relates generally to a system for automatically indicating or monitoring the positions of a reciprocally movable object, and more particularly it relates to such a system in which a rotary shaft encoder is used.
Many different systems have heretofore been proposed for detecting and tracking the angular position of a rotatable shaft. Such systems typically use multi-track rotary disc or drum encoders. The encoded tracks can be sensed or "read" either mechanically, optically, or magnetically. If at least two parallel tracks are used, with the encoded pattern of one of the tracks being staggered or out-of-phase with respect to the encoded pattern of a second track, logic means can be provided for identifying the direction of shaft rotation. See, e.g., U.S. Pat. No. 4,157,507.
In the prior art mechanical rotary encoder systems, each track comprises simply a rotatable cam having a plurality of lobes with which the actuating element of a relatively stationary electrical switch or interlock communicates, via a spring-loaded cam follower. In the prior art optical rotary encoders, each track is a series of alternately opaque and transparent segments on a rotatable disc; when aligned with a relatively stationary beam of light, the transparent segments are detected by an associated photo-sensitive element. In the prior art magnetic rotary encoders, each track of a rotary disc or drum is permanently magnetized to provide a series of alternately poled magnetic fields the polarity of which is sensed by a relatively stationary magnetic pickup or search coil adjacent to the track. An example of a magnetic rotary encoder is disclosed in U.S. Pat. No. 4,274,053. Hall effect devices are advantageously used as the polarity sensing elements (see U.S. Pat. No. 4,658,214).